Fish's Electric Organ Discharge Research Articles

The effect of urethane and MS-222 anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus.

Urethane and MS-222 are agents widely employed for general anesthesia, yet, besides inducing a state of unconsciousness, little is known about their neurophysiological effects. To investigate these effects, we developed an in vivo assay using the electric organ discharge (EOD) of the weakly electric fish Apteronotus leptorhynchus as a proxy for the neural output of the pacemaker nucleus. The oscillatory neural activity of this brainstem nucleus drives the fish's EOD in a one-to-one fashion. Anesthesia induced by urethane or MS-222 resulted in pronounced decreases of the EOD frequency, which lasted for up to 3h. In addition, each of the two agents caused a manifold increase in the generation of transient modulations of the EOD known as chirps. The reduction in EOD frequency can be explained by the modulatory effect of urethane on neurotransmission, and by the blocking of voltage-gated sodium channels by MS-222, both within the circuitry controlling the neural oscillations of the pacemaker nucleus. The present study demonstrates a marked effect of urethane and MS-222 on neural activity within the central nervous system and on the associated animal's behavior. This calls for caution when conducting neurophysiological experiments under general anesthesia and interpreting their results.

Dynamic Neuron-Glia Interactions in an Oscillatory Network Controlling Behavioral Plasticity in the Weakly Electric Fish, Apteronotus leptorhynchus

The involvement of glial cells in the regulation of physiological functions is being increasingly recognized, yet their role in plasticity of neural oscillators has remained largely elusive. An excellent model system to address the latter function is the pacemaker nucleus of the weakly electric fish, Apteronotus leptorhynchus. This brainstem oscillator drives the fish's electric organ discharge in a one-to-one fashion, with median frequencies of 880 Hz in males and 740 Hz in females. Morphometric analysis of the pacemaker nucleus has shown that astrocytes outnumber mature neurons seven-fold, and oscillator neurons even 200-fold. A similar dominance of astrocytes occurs among the adult-born cells that differentiate into glia and neurons. The astrocytes form a dense meshwork of cells interconnected by gap junctions. The degree of association of astrocytic fibers with the neural oscillator cells, and the gap-junction coupling between individual astrocytes, exhibit a sexual dimorphism, which parallels the sexual dimorphisms in the output frequency of the pacemaker nucleus, and ultimately in the electric organ discharge of the fish. It is hypothesized that the dynamics in astroglial structure mediate differences in the capacity to buffer potassium, which increases during the generation of action potentials. These differences, in turn, affect the excitability of the neural oscillator cells, and thus the output frequency of the pacemaker nucleus. Comparison of the pacemaker nucleus with other brain oscillators suggests that modulation of the output activity is one of the chief functions of the interaction of glia with the neural oscillator cells.

A temporal basis for predicting the sensory consequences of motor commands in an electric fish

Mormyrid electric fish are a model system for understanding how neural circuits predict the sensory consequences of motor acts. Medium ganglion cells in the electrosensory lobe create negative images that predict sensory input resulting from the fish's electric organ discharge (EOD). Previous studies have shown that negative images can be created through plasticity at granule cell-medium ganglion cell synapses, provided that granule cell responses to the brief EOD command are sufficiently varied and prolonged. Here we show that granule cells indeed provide such a temporal basis and that it is well-matched to the temporal structure of self-generated sensory inputs, allowing rapid and accurate sensory cancellation and explaining paradoxical features of negative images. We also demonstrate an unexpected and critical role of unipolar brush cells (UBCs) in generating the required delayed responses. These results provide a mechanistic account of how copies of motor commands are transformed into sensory predictions.

Responses to complex novel stimuli can be predicted by a simple neuron model of spike generation

Event Abstract Back to Event Responses to complex novel stimuli can be predicted by a simple neuron model of spike generation Henriette Walz1*, André Longtin2 and Jan Benda1 1 Ludwig-Maximilians-Universität München, Department Biologie II, Germany 2 University of Ottawa, Physics, Canada Sensory neurons have to encode stimuli that are drawn from distributions that considerably vary in their mean, standard deviation, spectral content, etc. depending on, for example, task or time of day. This potentially implies that a receptor neuron is driven in different dynamical regimes and that the neuron adapts on longer time scales to accommodate itself to such changes in the input statistics. Therefore, stimuli from all relevant statistics should be used when characterizing a neuron's responses. Alternatively, one can select a smaller set of stimulus classes for a characterization in form of, for example, a computational model and then probe different dynamical regimes of the neuron to challenge this characterization. We exemplify the latter approach on the example of electroreceptive neurons of weakly electric fish. Weakly electric fish use their electric organ discharge (EOD) for electrolocation as well as for communication. Amplitude modulations (AM) of the fish's EOD are encoded in electroreceptors (p-units). Behaviorally relevant AMs result in communication contexts from the interaction with other fish, or in hunting tasks from prey. The receptor cells respond phase-locked and probabilistically to each EOD cycle and show prominent negative serial correlations between successive interspike intervals. We built a simple leaky integrate-and-fire model with adaptation current (LIFAC) and constrained its operating point and intrinsic noise strength by the baseline statistics of measured p-units, while we set its sensitivity and adaptation properties to match responses to step stimuli. This model then not only reproduces the baseline ISI distributions and serial correlations, but also successfully predicts the spike responses to AMs from communication contexts. Interactions of two fish produce sinusoidal beats of a given frequency that are transiently modulated by communication signals. Therefore, the stimuli we used to probe our model, contain more complex dynamics than those the model is adjusted for. The generalizing properties of such a single cell model are based on the use of canonical models for the spike-generation and adaptation dynamics. By using established measurements to determine the parameters of the different processes, the model then captures the dynamics of the measured neuron for a large range of different stimulus classes. Therefore, we have shown that a simple model of spike generation can reproduce the behavior of a receptor cell to statistically different stimuli even when only constrained to its baseline statistics and step responses. Keywords: sensory processing Conference: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011, Freiburg, Germany, 4 Oct - 6 Oct, 2011. Presentation Type: Poster Topic: sensory processing (please use "sensory processing" as keyword) Citation: Walz H, Longtin A and Benda J (2011). Responses to complex novel stimuli can be predicted by a simple neuron model of spike generation. Front. Comput. Neurosci. Conference Abstract: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011. doi: 10.3389/conf.fncom.2011.53.00130 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 23 Aug 2011; Published Online: 04 Oct 2011. * Correspondence: Miss. Henriette Walz, Ludwig-Maximilians-Universität München, Department Biologie II, Martinsried, 82152, Germany, walz@bio.lmu.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Henriette Walz André Longtin Jan Benda Google Henriette Walz André Longtin Jan Benda Google Scholar Henriette Walz André Longtin Jan Benda PubMed Henriette Walz André Longtin Jan Benda Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Species-specific differences in sensorimotor adaptation are correlated with differences in social structure

Here, we report a species difference in the strength and duration of long-term sensorimotor adaptation in the electromotor output of weakly electric fish. The adaptation is produced by changes in intrinsic excitability in the electromotor pacemaker nucleus; this change is a form of memory that correlates with social structure. A weakly electric fish may be jammed by a similar electric organ discharge (EOD) frequency of another fish and prevents jamming by transiently raising its own emission frequency, a behavior called the jamming avoidance response (JAR). The JAR requires activation of NMDA receptors, and prolonged JAR performance results in long-term frequency elevation (LTFE) of a fish's EOD frequency for many hours after the jamming stimulus. We find that LTFE is stronger in a shoaling species (Eigenmannia virescens) with a higher probability of encountering jamming conspecifics, when compared to a solitary species (Apteronotus leptorhynchus). Additionally, LTFE persists in Eigenmannia, whereas, it decays over 5-9 h in Apteronotus.

A neural model of electrosensory system making electrolocation of weakly electric fish

We address the question of how the fish recognizes the location of objects at a given position, independently of the electric properties of objects, although the distortion pattern of electric field is varied depending on the electric property. To know how the electric field generated by fish's electric organ discharge (EOD) is spatially and temporally distorted by resistive and capacitive objects, we calculated the electric field using a reasonable model of fish body. P and T electroreceptors distributed on the fish body surface respond to the amplitude and phase modulations of EOD, respectively, and propagate their output signals to electrosensory lateral-line lobe (ELL) and torus semicircularis (TS), respectively. As the first step to clarify the neural mechanism of electrolocation, we made a neural network model of electrosensory system consisting of electroreceptors, afferent nerves, ELL, and TS, and investigated how the amplitude and phase modulations are encoded in ELL and TS, respectively.

The jamming avoidance response in Eigenmannia is controlled by two separate motor pathways

The gymnotiform fish Eigenmannia generates weakly electric signals for electrolocation and communication. The signals are produced by electric organ discharges (EODs) that are driven by a medullary pacemaker nucleus. To avoid jamming by neighboring conspecifics with similar frequencies, a fish raises its own EOD frequency if the neighbor's frequency is lower, and it lowers its EOD frequency if the neighbor's frequency is higher (Watanabe and Takeda, 1963). Both the raising and lowering of EOD frequency of this jamming avoidance response (JAR; Bullock et al., 1972) are thought to be controlled by feature-extracting neurons in the diencephalic prepacemaker nucleus (PPn-G) that discriminate the sign of the frequency difference between the jamming signal and the fish's EOD (Kawasaki et al., 1988a; Rose et al., 1988; for review, see Heiligenberg, 1991). These prepacemaker neurons are excited in response to lower jamming frequencies, thereby raising the frequency, and inhibited by higher jamming frequencies, producing a discharge deceleration. The results of experiments presented here, however, suggest a mechanism for the motor control of the JAR that is different from the one described previously (see, e.g., Heiligenberg, 1991). Two prepacemaker nuclei, one PPn-G and one sublemniscal prepacemaker nucleus (SPPn) (Keller et al., 1991 a,b), which provide the only known inputs to the pacemaker, were lesioned selectively. This article explores the effects of these lesions on the JAR. Pharmacological experiments were used to elucidate the transmitter types involved. The results suggest that the JAR is controlled by two separate motor pathways. One controls frequency rises and originates in the dorsal substructure of the nucleus electrosensorius (Keller, 1988). It sends excitatory connections to the diencephalic prepacemaker and finally to the pacemaker nucleus, where AMPA-type receptors mediate the synaptic transmission. The second pathway controls frequency decreases and originates in the ventral substructure of the nucleus electrosensorius. It provides GABAergic input to the SPPn. The SPPn is tonically active and also controls the EOD frequency even in the absence of jamming signals. Its projection to the pacemaker nucleus is mediated by NMDA-type receptors. The results of this study suggest that there is no single population of final, feature-extracting elements or "recognition units" that controls JAR-related shifts of the pacemaker frequency. Instead, the motor control of the JAR consists of an interaction of two independent pathways according to a "push-pull" principle.

An internal current source yields immunity of electrosensory information processing to unusually strong jamming in electric fish.

The electric organ of a fish represents an internal current source, and the largely isopotential nature of the body interior warrants that the current associated with the fish's electric organ discharges (EODs) recruits all electroreceptors on the fish's body surface evenly. Currents associated with the EODs of a neighbor, however, will not penetrate all portions of the fish's body surface equally and will barely affect regions where the neighbor's current flows tangentially to the skin surface. The computational mechanisms of the jamming avoidance response (JAR) in Eigenmannia exploit the uneven effects of a neighbor's EOD current to calculate the correct frequency difference between the two interfering EOD signals even if the amplitude of a neighbor's signal surpasses that of the fish's own signal by orders of magnitude. The particular geometry of the fish's own EOD current thus yields some immunity against the potentially confusing effects of unusually strong interfering EOD currents of neighbors.

Communication in the weakly electric fish Sternopygus macrurus. I. The neural basis of conspecific EOD detection.

There is a sexual dimorphism in the frequency of the quasi-sinusoidal electric organ discharge (EOD) of Sternopygus macrurus, with males, on average, an octave lower. EODs are detected by tuberous electroreceptor organs, which exhibit V-shaped frequency tuning with maximal sensitivity near the fish's own EOD frequency. This would seem to limit the ability of a fish to detect the EODs of opposite-sex conspecifics. However, electroreceptor tuning has always been based on single-frequency stimulation, while actual EOD detection involves the addition of a conspecific EOD to the fish's own. In the present study, recordings were made from single electroreceptive units while the fish were stimulated with pairs of sine waves: one (S1) representing the fish's own EOD added to a second (S2) representing a conspecific EOD. T unit response was easily predicted by assuming that the electroreceptor acts as a linear filter in series with a threshold-sensitive spike initiator. P unit response was more complex, and unexpectedly high sensitivity was found for frequencies of S2 well displaced from the fish's EOD frequency. For both P and T units, detection thresholds for S2 were much lower when added to S1, than when presented alone.

Discrimination of the sign of frequency differences bySternopygus, an electric fish without a jamming avoidance response

Several species of weakly electric fish reflexively change their frequency of electric organ discharge (EOD) in response to sensing signals of similar frequency from conspecifics; that is, they exhibit jamming avoidance responses (JAR).Eigenmannia increases its EOD frequency if jammed by a signal of lower frequency and decreases its EOD frequency if jammed by a signal of higher frequency. This discrimination is based on an analysis of the patterns of amplitude modulations and phase differences resulting from signal interference. Fish of the closely related genus,Sternopygus, however, do not exhibit a JAR. Here we show that despite lacking this behavior,Sternopygus shares many sensory processing capacities withEigenmannia: 1. Fish could be conditioned to discriminate the sign of the frequency difference (Df) between an exogenous sinusoidal signal and its own EODs with as few as 300 training trials. 2. Fish can discriminate the sign of Df for jamming signals with an amplitude as low as 2 μV/cm (p-p); which is approximately 40 dB below the amplitude of the fish's EOD, as measured lateral to the operculum. 3. Fish appear to discriminate the sign of Df by evaluating modulations in signal amplitude and differences in the timing of signals received by different areas of the body surface. 4. Sternopygus processes electrosensory information through band-pass modulation filters. Tuning to modulation rates over at least 4–16 Hz exists.

Stimulus discrimination in the diencephalon ofEigenmannia: the emergence and sharpening of a sensory filter

Neuronal reliability and sensitivity to behaviorally relevant stimulus patterns were investigated in a higher-order nucleus of the diencephalon believed to participate in the jamming avoidance response (JAR) of the weakly electric fish, Eigenmannia. The fish raises or lowers its frequency of electric organ discharge (EOD) to minimize interference from a neighboring fish's EOD. Proper JARs require determination of the sign of the difference frequency (Df) between the neighboring fish's EOD and the fish's own EOD. Bastian and Yuthas (1984) recently described diencephalic neurons within the nucleus electrosensorius that are able to make this determination. In the present study, response properties of such neurons were compared with those of lower-level 'sign-selective' cells found in the torus semicircularis and the optic tectum (Heiligenberg and Rose 1985) as well as with properties of the intact behavior. Most sign-selective cells within the nucleus electrosensorius show a high degree of selectivity for one sign of the difference frequency; cells with either sign preference were found in approximately equal numbers. The sign preference and the degree of sign selectivity is most often independent of the spatial orientation of the jamming stimulus. In contrast, the responses of toral and tectal cells are less robust and consistent and are often highly dependent on the geometry of the jamming stimulus. Determination of the sign of the difference frequency requires the analysis of amplitude modulations coupled with modulations in phase (timing) differences between pairs of areas of the body surface. The most sensitive cells recorded in the nucleus electrosensorius can determine the sign of the difference frequency with timing differences of 1 microsecond or less, roughly comparable to the behavioral threshold of 400 ns (Carr et al. 1986). The best toral/tectal response required at least a 16 microseconds modulation. Cells within the nucleus electrosensorius thus code the sign of Df with a high degree of reliability and sensitivity. Ambiguities persist, however, which suggest that single cells at this level cannot completely account for the behavioral discrimination. Additional processing may be necessary to transform a still primarily sensory code into a motor program for control of the JAR (Rose et al. 1988).

Sex recognition and neuronal coding of electric organ discharge waveform in the pulse-type weakly electric fish, Hypopomus occidentalis.

1. Hypopomus occidentalis, a weakly electric gymnotiform fish with a pulse-type discharge, has a sexually dimorphic electric organ discharge (Hagedorn 1983). The electric organ discharges (EODs) of males in the breeding season are longer in duration and have a lower peak-power frequency than the EODs of females. We tested reproductively mature fish in the field by presenting electronically generated stimuli in which the only cue for sex recognition was the waveshape of individual EOD-like pulses in a train. We found that gravid females could readily discriminate male-like from female-like EOD waveshapes, and we conclude that this feature of the electric signal is sufficient for sex recognition. 2. To understand the possible neural bases for discrimination of male and female EODs by H . occidentalis, we conducted a neurophysiological examination of both peripheral and central neurons. Our studies show that there are sets of neurons in this species which can discriminate male or female EODs by coding either temporal or spectral features of the EOD. 3. Temporal encoding of stimulus duration was observed in evoked field potential recordings from the magnocellular nucleus of the midbrain torus semicircularis. This nucleus indirectly receives pulse marker electroreceptor information. The field potentials suggest that comparison is possible between pulse marker activity on opposite sides of the body. 4. From standard frequency-threshold curves, spectral encoding of stimulus peak-power frequency was measured in burst duration coder electroreceptor afferents. In both male and female fish, the best frequencies of the narrow-band population of electroreceptors were lower than the peak-power frequency of the EOD. Based on this observation, and the presence of a population of wide-band receptors which can serve as a frequency-independent amplitude reference, a slope-detection model of frequency discrimination is advanced. 5. Spectral discrimination of EOD peak-power frequency was also shown to be possible in a more natural situation similar to that present during behavioral discrimination. As the fish's EOD mimic slowly scanned through and temporally coincided with the neighbor's EOD mimic, peak spike rate in burst duration coder afferents was measured. Spike rate at the moment of coincidence changed predictably as a function of the neighbor's EOD peak-power frequency. 6. Single-unit threshold measurements were made on afferents from peripheral burst duration coder receptors in the amplitude-coding pathway, and midbrain giant cells in the time-coding pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamics and stimulus-dependence of pacemaker control during behavioral modulations in the weakly electric fish, Apteronotus.

1. Weakly electric fish generate around their bodies low-amplitude, AC electric fields which are used both for the detection of objects and intraspecific communication. The types of modulation in this signal of which the high-frequency wave-type gymnotiform, Apteronotus, is capable are relatively few and stereotyped. Chief among these is the chirp, a signal used in courtship and agonistic displays. Chirps are brief and rapid accelerations in the normally highly regular electric organ discharge (EOD) frequency. 2. Chirping can be elicited artificially in these animals by the use of a stimulus regime identical to that typically used to elicit another behavior, the jamming avoidance response (JAR). The neuronal basis for the JAR, a much slower and lesser alteration in EOD frequency, is well understood. Examination of the stimulus features which induce chirping show that, like the JAR, there is a region of frequency differences between the fish's EOD and the interfering signal that maximally elicits the response. Moreover, the response is sex-specific with regard to the sign of the frequency difference, with females chirping preferentially on the positive and most males on the negative Df. These features imply that the sensory mechanisms involved in the triggering of these communicatory behaviors are fundamentally similar to those explicated for the JAR. 3. Additionally, two other modulatory behaviors of unknown significance are described. The first is a non-selective rise in EOD frequency associated with a JAR stimulus, occurring regardless of the sign of the Df. This modulation shares many characteristics with the JAR. The second behavior, which we have termed a 'yodel', is distinct from and kinetically intermediate to chirping and the JAR. Moreover, unlike the other studied electromotor behaviors it is generally produced only after the termination of the eliciting stimulus.

Coordination of EOD frequency and pulse duration in a weakly electric wave fish: the influence of androgens

1. The electric organ discharge (EOD) of wave-type weakly electric fish is generated as an extremely regular series of electric organ pulses. Measurements of pulse duration and EOD frequency were made in the speciesSternopygus andEigenmannia. Pulse duration is highly inversely correlated with EOD frequency in a population of fish, so that the EOD waveform remains quasisinusoidal over the species range of EOD frequencies. This places most of the energy of the EOD within the fundamental harmonic to which the electroreceptors are most sensitive. 2. Treatment with the anesthetic MS-222 transiently lowers EOD frequency, but does not change pulse duration. This demonstrates that pulse duration is independent of immediate EOD frequency and, therefore, of the medullary pacemaker nucleus (PMN). 3. InSternopygus, small monophasic potentials could be recorded outside the tails of curarized fish. These were of constant duration and shape across all fish regardless of the fish's EOD frequency. The location along the body where this potential reversed polarity was close to, although not identical with, the isopotential line for the EOD pulse. These potentials were partially blocked by the curare. They probably represent the summed psps of the synchronously-firing electrocytes. This result supports the hypothesis that differences in EOD waveform, specifically pulse duration, arise from electrocyte membrane events following the psp (postsynaptic potential). 4. Implantation ofSternopygus with 5-α-dihydrotestosterone (DHT) in silastic capsules results in decreased EOD frequencies, as previously reported for this species (Meyer and Zakon 1982; Meyer 1983). In addition, corresponding significant increases in EOD pulse duration occurred (a mean increase of about 1.3 ms or 24%). No changes were measured in controls implanted with empty capsules. The maximum decrease in EOD frequency and increase in pulse duration plateau at different times, suggesting that they are the result of different processes. After removal of the hormone capsules, EOD frequency and pulse duration reverted to baseline levels within a few weeks. 5. These results demonstrate a degree of coordination between the PMN and the electric organ not previously recognized. It is likely that androgens play a role in this process. If, as proposed, the electrocyte membrane properties determine the characteristics of the waveform, then the androgen must act on both the PMN and the electrocytes to keep EOD frequency and pulse duration in register.

The emergence of tuning in newly generated tuberous electroreceptors

Tuning curves of afferent electroreceptive fibers in the anterior lateral line nerve of the weakly electric fish, Sternopygus macrurus, indicate that the tuberous electroreceptors of each individual are well-tuned to its own electric organ discharge (EOD) frequency. In order to study how receptor tuning may develop, new receptor organs were induced to form in regenerating cheek skin, and their tuning properties were compared with those of intact receptors from the same fish. At 3 weeks after the onset of regeneration, new receptors of a given fish were broadly tuned with best frequencies (BFs) lower than that fish's EOD frequency and the BFs of its own intact tuberous receptors. Three weeks later, regenerated receptors of the same fish were indistinguishable from intact receptors in BF, although tuning curves were occasionally slightly broader than normal. To determine if the presence of an ongoing electric field is necessary for the genesis of proper tuning, receptors were allowed to regenerate in fish deprived of their EODs. At 6 weeks, tuning curves of these receptors also had BFs that were tuned similarly to intact receptors and to each individual's characteristic EOD frequency (determined by recordings of the pacemaker nucleus in the medulla). Thus, as regenerating receptors mature, they gradually become more sharply tuned and tuned to progressively higher frequencies until reaching the correct BF, which matches the EOD frequency; however, tuning to the appropriate EOD frequency occurs without reference to the ongoing electric field.

Neural coding of difference frequencies in the midbrain of the electric fish Eigenmannia: reading the sense of rotation in an amplitude-phase plane.

Eigenmannia is able to discriminate the sign of the difference, Df, between the frequency of a neighbor's electric organ discharge (EOD) and that of its own EOD. This discrimination can be demonstrated at the level of individual neurons of the midbrain. Intracellular and extracellular recordings of such sign-selective cells revealed the following: Units preferring positive Dfs and units preferring negative Dfs were found with equal frequency. The degree of selectivity was also similar for these two classes of neurons. All sign-selective units were sensitive to the magnitude of the frequency difference, i.e. the beat rate. Most units responded best to beat rates in the 4-8 Hz range. Sign-selectivity was observed only when the jamming signal (S2) was presented through electrodes other than those used to deliver the mimic (S1) of the fish's EOD, i.e. only when amplitude modulations were accompanied by modulations of differential phase. Intracellular studies suggest that most sign-selective neurons of the tectum are large, multipolar cells in the stratum album centrale. These cells send projections to the reticular formation, to lamina 9 of the torus semicircularis and to the N. electrosensorius.

Evidence for a direct effect of androgens upon electroreceptor tuning.

Tuberous electroreceptors of individual wave type weakly electric fish are tuned to the fundamental frequency of that fish's electric organ discharge (EOD). EOD frequency and receptor best frequency (BF) are both lowered following systemic injection of 5-alpha-dihydrotestosterone (DHT). A previous study (Meyer et al. 1984) showed that the effect of DHT on the EOD generating circuitry was independent of an ongoing EOD and suggested that its effect on electroreceptor tuning was indirect, possibly mediated by the electric field. We have continued these studies to determine the factors which influence electroreceptor tuning. Baseline recordings of EOD frequency, receptor oscillations, and single afferent tuning curves were taken. After fish were electrically silenced by spinal cord transection they were injected daily with either DHT or saline or were implanted with either DHT-filled or empty silastic capsules. As previously reported, the EOD frequency (determined from pacemaker nucleus recordings) was lowered in DHT-treated, transected fish and increased in control fish. Similarly, receptor tuning was lowered in the DHT-treated, silenced fish. Oscillation frequencies decreased in both treated and control groups, but significantly more in the hormone group. Single afferent best frequencies were lowered in both DHT groups and raised in their respective control groups. In another series of experiments exogenous electric fields capable of driving receptors in a 1-to-1 phase-locked manner were placed around silenced fish. We were unable to elicit any shift in pacemaker frequency or electroreceptor tuning regardless of stimulus field geometry. Four transected fish were injected with DHT and placed in exogenous electric fields of higher frequency than their original EOD. Even in the presence of a higher frequency electric field, DHT lowered EOD frequency and afferent BF. We conclude that androgens produce effects both on the EOD generating circuitry, probably at the level of the pacemaker nucleus, and on electroreceptors, probably, ultimately, on receptor cell membrane conductances. These effects occur in parallel allowing the two parameters to remain well matched. In contrast to former predictions, exogenous electric fields alone appear unable to shift receptor tuning.

Sensory control of pacemaker acceleration and deceleration in gymnotiform electric fish with pulse-type discharges

South American pulse species of weakly electric fish change their frequency of electric organ discharges (EODs) to avoid jamming of their electrolocation sense by other fish at similar frequencies (Fig. 1). If a ‘foreign’ fish is at a lower frequency its pulses scan from negative to positive latencies with respect to the fish's EOD, resulting in a strong acceleration in EOD frequency. Pulses of another fish at a higher frequency scan from positive to negative latencies, evoking deceleration, or only a very small acceleration. Thus the jamming avoidance response (JAR) takes the form of a differential response to opposite scan directions.

The effect of temperature on a social behavior of weakly electric fish Eigenmannia virescens

1. 1. The frequency of the electric organ discharge (EOD) of Eigenmannia virescen changes with water temperature and with the presence of an electrical sine wave stimulus of a neighboring frequency. The latter response, called the jamming avoidance response, was observed at various temperatures ranging from 18 to 30°C. 2. 2. Jamming avoidance response (JAR) in Eigenmannia, characterized by a shift in the fish's EOD frequency away from the electrical stimulus frequency, increased with rising temperature. 3. 3. The Q 10 of the resting EOD frequency and JAR were different in their absolute values and variation patterns with temperature. 4. 4. These results suggest that the neuronal mechanisms for the 2 behavioral responses are probably independent and the integration site is likely to be at the pacemaker cells.

Behavioral thresholds to electric signals in high frequency electric fish

1) The behavioral thresholds ofApteronotus andEigenmannia to imposed sine wave electric fields were measured as a function of stimulus frequency using operant conditioning techniques. 2)Apteronotus responds to a broad range of frequencies. Its threshold—frequency curve is complex, showing three distinct threshold dips at: <5–15 Hz, 0.6–0.9 μV/cm; 200–300 Hz, 0.9 μV/cm; and 700–2200 Hz to 0.2 μV/cm. The high frequency dip itself is bimodal, with a primary minimum at the fish's electric organ discharge (EOD) fundamental, and a secondary minimum at its second harmonic. 3) The threshold—frequency curve forEigenmannia differs in several respects, being unimodal and sharpely tuned to the fish's EOD frequency with a sensitivity of 0.2–0.3 μV/cm.Eigenmannia shows a lack of keen low frequency perception, and no marked sensitivity increase at the second harmonic of its EOD. 4) The EOD wave forms ofApteronotus andEigenmannia were Fourier analysed:Eigenmannia's EOD approximates a sine wave, whileApteronotus' EOD contains considerable power at harmonic frequencies. 5) It is proposed that behavioral thresholds to frequencies within ca. 50–100 Hz of the fish's EOD fundamental or second harmonic are improved by the fish's extraction of, and preference for, low frequency beats of its EOD with the stimulus. This is suggested by the sharp, EOD specific tuning demonstrated by bothEigenmannia andApteronotus and the second harmonic sensitivity peak ofApteronotus, which are not predicted by known receptor response characteristics. Such a beat extraction capability might provide a mechanism for improving the resolution of conspecific electrocommunication. 6) The influence of water resistivity on the behavioral thresholds ofApteronotus was measured. High frequency sensitivity increases as a power function with decreasing water resistivity down to 1 k Ohm · cm, below which it remains constant. Low frequency sensitivity increases progressively from 100 k Ohm · cm down to 10 k Ohm · cm, but decreases again below this value. Thus low frequency eleotroreceptors are maximally sensitive in 10 k Ohm · cm water, while high frequency receptors function best in water below 1 k Ohm · cm.